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Data and Computer Communications

Data and Computer Communications. Chapter 5 – Signal Encoding Techniques . Eighth Edition by William Stallings Lecture slides by Lawrie Brown. Signal Encoding Techniques. Even the natives have difficulty mastering this peculiar vocabulary —The Golden Bough , Sir James George Frazer.

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Data and Computer Communications

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  1. Data and Computer Communications Chapter 5 – Signal Encoding Techniques Eighth Edition by William Stallings Lecture slides by Lawrie Brown

  2. Signal Encoding Techniques Even the natives have difficulty mastering this peculiar vocabulary —The Golden Bough, Sir James George Frazer

  3. Signal Encoding Techniques

  4. Digital Data, Digital Signal • Digital signal • discrete, discontinuous voltage pulses • each pulse is a signal element • binary data encoded into signal elements

  5. Some Terms • unipolar • polar • data rate • duration or length of a bit • modulation rate • mark and space

  6. Interpreting Signals • need to know • timing of bits - when they start and end • signal levels • factors affecting signal interpretation • signal to noise ratio • data rate • bandwidth • encoding scheme

  7. Comparison of Encoding Schemes • signal spectrum • clocking • error detection • signal interference and noise immunity • cost and complexity

  8. Comparison (1) • Signal Spectrum • Lack of high frequencies reduces required bandwidth • Lack of dc component allows ac coupling via transformer, providing isolation • Concentrate power in the middle of the bandwidth • Clocking • Synchronizing transmitter and receiver • External clock • Sync mechanism based on signal

  9. Comparison (2) • Error detection • Can be built in to signal encoding • Signal interference and noise immunity • Some codes are better than others • Cost and complexity • Higher signal rate (& thus data rate) lead to higher costs • Some codes require signal rate greater than data rate

  10. Encoding Schemes

  11. Nonreturn to Zero-Level(NRZ-L) • two different voltages for 0 and 1 bits • voltage constant during bit interval • no transition I.e. no return to zero voltage • such as absence of voltage for zero, constant positive voltage for one • more often, negative voltage for one value and positive for the other

  12. Nonreturn to Zero Inverted • nonreturn to zero inverted on ones • constant voltage pulse for duration of bit • data encoded as presence or absence of signal transition at beginning of bit time • transition (low to high or high to low) denotes binary 1 • no transition denotes binary 0 • example of differential encoding since have • data represented by changes rather than levels • more reliable detection of transition rather than level • easy to lose sense of polarity

  13. NRZ Pros & Cons • Pros • easy to engineer • make good use of bandwidth • Cons • dc component • lack of synchronization capability • used for magnetic recording • not often used for signal transmission

  14. Multilevel BinaryBipolar-AMI • Use more than two levels • Bipolar-AMI • zero represented by no line signal • one represented by positive or negative pulse • one pulses alternate in polarity • no loss of sync if a long string of ones • long runs of zeros still a problem • no net dc component • lower bandwidth • easy error detection

  15. Multilevel BinaryPseudoternary • one represented by absence of line signal • zero represented by alternating positive and negative • no advantage or disadvantage over bipolar-AMI • each used in some applications

  16. Multilevel Binary Issues • synchronization with long runs of 0’s or 1’s • can insert additional bits, cf ISDN • scramble data (later) • not as efficient as NRZ • each signal element only represents one bit • receiver distinguishes between 3 levels: +A, -A, 0 • 3 level system could represent log23 = 1.58 bits • requires approx. 3dB more signal power for same probability of bit error

  17. Manchester Encoding • has transition in middle of each bit period • transition serves as clock and data • low to high represents one • high to low represents zero • used by IEEE 802.3

  18. Differential Manchester Encoding • midbit transition is clocking only • transition at start of bit period representing 0 • no transition at start of bit period representing 1 • this is a differential encoding scheme • used by IEEE 802.5

  19. Biphase Pros and Cons • Con • at least one transition per bit time and possibly two • maximum modulation rate is twice NRZ • requires more bandwidth • Pros • synchronization on mid bit transition (self clocking) • has no dc component • has error detection

  20. Modulation Rate

  21. (Modulation Rate) / (Data Rate)

  22. Scrambling • use scrambling to replace sequences that would produce constant voltage • these filling sequences must • produce enough transitions to sync • be recognized by receiver & replaced with original • be same length as original • design goals • have no dc component • have no long sequences of zero level line signal • have no reduction in data rate • give error detection capability

  23. B8ZS and HDB3 if even if odd Violation within the substituted code

  24. B8ZS: Bipolar with 8-zeros substitution • if an octet of all zeros occurs and the last voltage pulse preceding this octet was positive, then the eight zeros of the octet are encoded as 000+-0-+ • if an octet of all zeros occurs and the last voltage pulse preceding this octet was negative, then the eight zeros of the octet are encoded as 000-+0+-

  25. HDB3: High-density bipolar-3 zeros 1 2 1 1 2 2 1 2 Check if you know why DC is still zero!

  26. Digital Data, Analog Signal • main use is public telephone system • has freq range of 300Hz to 3400Hz • use modem (modulator-demodulator) • encoding techniques • Amplitude shift keying (ASK) • Frequency shift keying (FSK) • Phase shift keying (PSK)

  27. Modulation Techniques

  28. Amplitude Shift Keying • encode 0/1 by different carrier amplitudes • usually have one amplitude zero • susceptible to sudden gain changes • inefficient • used for • up to 1200bps on voice grade lines • very high speeds over optical fiber

  29. Binary Frequency Shift Keying • most common is binary FSK (BFSK) • two binary values represented by two different frequencies (near carrier) • less susceptible to error than ASK • used for • up to 1200bps on voice grade lines • high frequency radio • even higher frequency on LANs using co-ax

  30. Multiple FSK • each signalling element represents more than one bit • more than two frequencies used • more bandwidth efficient • more prone to error

  31. Phase Shift Keying • phase of carrier signal is shifted to represent data • binary PSK • two phases represent two binary digits • differential PSK • phase shifted relative to previous transmission rather than some reference signal

  32. Quadrature PSK • get more efficient use if each signal element represents more than one bit • eg. shifts of /2 (90o) • each element represents two bits • split input data stream in two & modulate onto carrier & phase shifted carrier • can use 8 phase angles & more than one amplitude • 9600bps modem uses 12 angles, four of which have two amplitudes

  33. QPSK and OQPSK Modulators PSK PSK

  34. Performance of Digital to Analog Modulation Schemes • bandwidth • ASK/PSK bandwidth directly relates to bit rate • multilevel PSK gives significant improvements • in presence of noise: • bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK • for MFSK & MPSK have tradeoff between bandwidth efficiency and error performance

  35. Quadrature Amplitude Modulation • QAM used on asymmetric digital subscriber line (ADSL) and some wireless • combination of ASK and PSK • logical extension of QPSK • send two different signals simultaneously on same carrier frequency • use two copies of carrier, one shifted 90° • each carrier is ASK modulated • two independent signals over same medium • demodulate and combine for original binary output

  36. QAM Modulator ASK ASK

  37. QAM Variants • two level ASK • each of two streams in one of two states • four state system • essentially QPSK • four level ASK • combined stream in one of 16 states • have 64 and 256 state systems • improved data rate for given bandwidth • but increased potential error rate

  38. Analog Data, Digital Signal • digitization is conversion of analog data into digital data which can then: • be transmitted using NRZ-L • be transmitted using code other than NRZ-L • be converted to analog signal • analog to digital conversion done using a codec • pulse code modulation • delta modulation

  39. Digitizing Analog Data

  40. Pulse Code Modulation (PCM) • sampling theorem: • “If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all information in original signal” • eg. 4000Hz voice data, requires 8000 sample per sec • strictly have analog samples • Pulse Amplitude Modulation (PAM) • so assign each a digital value

  41. PCM Example

  42. PCM Block Diagram

  43. Non-Linear Coding

  44. Companding

  45. Delta Modulation • analog input is approximated by a staircase function • can move up or down one level () at each sample interval • has binary behavior • since function only moves up or down at each sample interval • hence can encode each sample as single bit • 1 for up or 0 for down

  46. Delta Modulation Example

  47. Delta Modulation Operation

  48. PCM verses Delta Modulation • DM has simplicity compared to PCM • but has worse SNR • issue of bandwidth used • eg. for good voice reproduction with PCM • want 128 levels (7 bit) & voice bandwidth 4khz • need 8000 x 7 = 56kbps • data compression can improve on this • still growing demand for digital signals • use of repeaters, TDM, efficient switching • PCM preferred to DM for analog signals

  49. Analog Data, Analog Signals • modulate carrier frequency with analog data • why modulate analog signals? • higher frequency can give more efficient transmission • permits frequency division multiplexing (chapter 8) • types of modulation • Amplitude • Frequency • Phase

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